Optical communication system

ABSTRACT

A transmitting apparatus includes a plurality of code spreaders different in spreading code, a reception processing unit that selectively distributes transmission data to the plurality of code spreaders, a plurality of optical transmitters each of which that transmit a code-spread signal to an optical fiber as a CDMA optical signal of a carrier wavelength different from that of the other optical transmitters, and a signal multiplexing unit that selectively supplies outputs of the plurality of code spreaders to the plurality of optical transmitters, and a receiving apparatus includes an optical receiver that receives a wavelength-division-multiplexed CDMA optical signal from the optical fiber, and a plurality of despreaders connected to the optical receiver and different in spreading code, wherein each of the despreaders reproduces a CDMA signal corresponding to its spreading code from an output signal of the optical receiver.

TECHNICAL FIELD

The present invention relates to an optical communication system, andmore particularly to a multiplexed optical signal transmission systemthat enables receiving of optical transmission signals multiplexed on anoptical fiber by a single optical receiving device.

BACKGROUND ART

Services on a network are diversified and new services taking advantageof the network are expanding. As a representative example, there is amerged service of broadcasting and communication, that is, anintegration of broadcasting, Internet, and telephone (voicecommunication) services called triple services. This service is arepresentative application of existing information services, and tripleplay has become a keyword indicating the next-generation network conceptto accommodate diversified information communication services.

In such circumstances, in access networks, construction of FTTH by PON(Passive Optical Network) becomes the mainstream. A PON system comprisesan office side apparatus OLT (Optical Line Terminal) located in theoffice building of a communication carrier, and a plurality ofsubscriber connecting apparatus ONTs (Optical Network Terminals) eachbeing located at a user home. Signals are distributed to individualhomes in a point-to-multipoint form, by laying a single optical fiber(trunk optical fiber) from the OLT to a service area, diverging thetrunk optical fiber into a plurality of branched optical fibers by asplitter and connecting each branched optical fiber to the ONT.

Since the PON system has the function of multicasting signals throughoptical branching, it is useful, for example, as an infrastructure fordistributing large-capacity of data such as high-resolution images.Further, as a plurality of OLTs can share the trunk optical fiber, thePON system has an advantage that the costs of laying optical fibers andthe number of transmitting/receiving devices in the OLT side can bereduced in comparison with a star-type connection in which the officebuilding and each user home are connected in a point-to-point manner.Current PON systems include G (Gigabit-capable)-PON of ITU-T standard(Non-patent Documents 1 to 3) and GE (Gigabit-Ethernet)-PON of IEEEstandard (Non-patent Document 4).

During the expansion of merged services of broadcasting andcommunication is attracting attention, a further increase incommunication density (higher-level multiplexing), an increase incommunication speed (high bit rate), and expansion of fiber laying areaare demanded for PON systems in order to distribute high-resolutionimages such as, for example, high definition TV to a large number ofusers. The standardization group (IUT-T and IEEE) related to PON isstarting a study of the next-generation PON to be a successor to currentPON systems.

Presently, 10GE-PON and WDM-PON are proposed in these standardizationconferences as next-generation PON. As a multiplexing method fornext-generation PON, time division multiple access (TDMA) is mainstreamlike the current PON, and application of code division multiple access(CDMA) is being studied as another multiplexing method. CDMA has theadvantage that it has higher information transmission efficiency pertransmission bandwidth than TDMA, because it enables concurrenttransmission/reception of a plurality of flows with the same carrier,and adjustment of communication timing among ONTs and the securement ofguard time between frames are unnecessary. Moreover, since CDMA canprotect transmission data by spread spectrum with orthogonal spreadingcode, increasing the secrecy of information can be expected in PONsystems that accommodates a large number of users on a single opticalfiber.

Non-patent Document 1: ITU-T G.984.1 “Gigabit-capable Passive OpticalNetworks (GPON): General characteristics”

Non-patent Document 2: ITU-T G.984.2 “Gigabit-capable Passive OpticalNetworks (GPON): Physical Media Dependent (PMD) layer specification”

Non-patent Document 3: ITU-T G.984.3 “Gigabit-capable Passive OpticalNetworks (GPON): Transmission convergence layer specification”

Non-patent Document 4: IEEE 802.3ah “CSMA/CD Access Method and PhysicalLayer Specification Amendment: Media Access Control Parameters, PhysicalLayers, and Management Parameters for Subscriber Access Networks”

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In optical CDMA, when CDMA signals are transmitted from a plurality ofcommunication apparatuses to the same optical fiber by using carriers(laser beams) of the same wavelength, mutual interference occurs amongtransmission signals. If two optical transmission signals exist on theoptical fiber in a wholly reverse phase with each other, the opticalsignals are cancelled out by mutual interference and signal strengthbecomes zero, with the result that transmission information is whollylost (homodyne interference). Even when carriers different in wavelengthare used, mutual interference occurs between transmission signals in thecase where the wavelength difference between the carriers isinsufficient. This mutual interference is called beat noise (heterodyneinterference).

The homodyne interference and the heterodyne interference occur also inthe CDMA mobile wireless communication. However, in the case of mobilewireless communication, because the positional relationship between abase station and mobile terminals varies with time, even if theabove-described mutual interference occurs between transmission signals,the influence is momentarily and trivial in comparison with theinfluence of noise level and multipath.

However, in an optical access network such as PON, since the positionalrelationship between OLT and ONT is fixed and a transmitting apparatustransmits a signal with a stable laser beam, the influence may last fora long period of time if the above-described interference occurs on theoptical fiber.

An object of the present invention is to provide an opticalcommunication system capable of avoiding mutual interference oftransmission signals on the optical fiber and enable transmission ofhighly multiplexed signals.

Another object of the present invention is to provide an opticalcommunication system of PON configuration that enables transmission ofhighly multiplexed signals by using code division multiplexing.

Means for Solving the Problem

In order to achieve the above-described objects, an opticalcommunication system of the present invention is characterized in thatCDMA signals are multiplexed on an optical fiber by WDMA. In the opticalcommunication system of the present invention, a different carrierwavelength (laser wavelength) is used for each optical transmitter inorder to suppress signal deterioration due to interference of laserbeams. Further, allocation of carrier wavelength to each opticaltransmitter is performed within an optical frequency band (wavelengthrange) receivable by a single optical receiving device, and the intervalbetween adjacent carrier wavelengths is made larger than the frequencybandwidth of a CDMA signal receivable by the optical receiving device sothat interference components are prevented from being detected at areceiving side.

More specifically, an optical communication system of the presentinvention comprises a transmitting apparatus and a receiving apparatusconnected to each other by an optical fiber. The transmitting apparatuscomprises a plurality of code-division spreaders different in spreadingcode, a reception processing unit that selectively distributestransmission data to the plurality of code-division spreaders, aplurality of optical transmitters each of which transmits a code-spreadsignal to the optical fiber as a CDMA optical signal of a carrierwavelength different from that of the other optical transmitters, and asignal multiplexing unit that selectively supplies outputs of theplurality of code-division spreaders to the plurality of opticaltransmitters. The receiving apparatus comprises an optical receiver thatreceives a wavelength-division-multiplexed CDMA optical signal from theoptical fiber, and a plurality of despreaders connected to the opticalreceiver and different in their spreading codes, wherein each ofdespreaders reproduces a CDMA signal corresponding to its spreading codefrom an output signal of the optical receiver.

An optical communication system of PON configuration according to thepresent invention comprises an office side apparatus (OLT) connected toa trunk optical fiber, and a plurality of subscriber connectingapparatuses (ONT) connected to branched optical fibers diverged from thetrunk optical fiber. Each of the ONTs comprises a plurality ofcode-division spreaders different in their spreading codes, a receptionprocessing unit that selectively distributes transmission data to theplurality of code-division spreaders, at least one optical transmitterthat transmits a code-spread signal to the branched optical fiber as anoptical signal of a carrier wavelength different from that of the otheroptical transmitters, and a signal multiplexing unit that selectivelysupplies outputs of the plurality of code-division spreaders to theoptical transmitter. The OLT comprises an optical receiver that receivesa wavelength-division-multiplexed CDMA signal from the trunk opticalfiber, and a plurality of despreaders connected to the optical receiverand different in their spreading codes, wherein each of the despreadersreproduces a CDMA signal corresponding to its spreading code from anoutput signal of the optical receiver.

In the case of the optical communication system of PON configuration,each of the ONTs converts transmission data into a CDMA signal by thecode-division spreader having a spreading code allocated from the OLT,and converts the CDMA signal into an optical signal by the opticaltransmitter having a carrier wavelength allocated from the OLT.

In an embodiment of the present invention, each of the ONTs includes aplurality of optical transmitters different in their carrierwavelengths. However, only a part of the plurality of opticaltransmitters may be made effective so that the rest are standby opticaltransmitters. In this case, the signal multiplexing unit selectivelysupplies outputs of the plurality of code-division spreaders to at leastone optical transmitter in the effective state. A laser elementadjustable its oscillation wavelength can be applied to each opticaltransmitter. In this case, each of the ONTs controls the wavelength ofeach laser element so that each of the optical transmitters in theeffective state has a carrier wavelength allocated in advance.

In an embodiment of the present invention, each of the ONTs has awavelength allocation table indicating the correspondence betweenspreading codes and carrier wavelengths, and the signal multiplexingunit correlates the code-division spreaders and optical transmittersaccording to the wavelength allocation table. Further, each of the ONTshas a flow identifier table indicating the correspondence between flowidentifiers of transmission data and spreading codes, and the receptionprocessing unit correlates the transmission data and the code-divisionspreader according to the flow identification table.

Effect of the Invention

According to the present invention, by using the principle of WDM incombination with optical CDMA, the interference of lightwaves (influenceof beat noise) impeding the practical use of optical CDMA can beavoided. Moreover, each ONT can transmit data in any access timing byapplying CDMA and effectively using transmission capacity of opticalfiber, without needing synchronization between ONTs unlike conventionalPON systems.

BEST MODE FOR CARRYING OUT THE PREFERRED EMBODIMENTS

FIG. 1 shows the construction of a PON system to which optical CDMA ofthe present invention is applied, and carrier wavelengths (carrierfrequencies) in optical fiber sections.

The PON system comprises an office side apparatus OLT (Optical LineTerminal) 1 and a plurality of subscriber connecting apparatuses ONT(Optical Network Terminal) 2 (2-1 to 2-k) connected to the OLT 1 throughan optical fiber network. The optical fiber network comprises trunkoptical fibers 700 (700-1 to 700-m) connected to the OLT 1, and aplurality of branched fibers 710 (710-1 to 710-k) connected to the trunkoptical fiber 700 by optical splitters (optical couplers) 800 (800-1 to800-m). Each ONT 2 is connected to the branched fiber 710, and aplurality of ONTs share the same trunk optical fiber 700 tocommunication with the OLT 1.

To each ONT 2, one or a plurality of user terminals 500 are connected.The user terminals 500 can be connected to the ONT in various forms. Forexample, a terminal 500-2 is connected to the ONT through an individualsubscriber line. There is a case of connecting to the ONT through a homeswitch or home router 300, like terminals 500-1A and 500-1N, or a caseof connecting through a home network (LAN) 400, like a terminal 500-k.In the following description, a PC or a server to be arranged in anenterprise site for use in business as well as a personal PC to be usedin an individual home will be referred to as the user terminal 500.

Each ONT 2, for example, as shown in ONT 2-k, comprises a switch 21accommodating connection lines for user terminals, an optical interface22 connected to a branched fiber 710, an ONT upward interface board(hereinafter referred to as ONT-UIF) 20U and an ONT downward interfaceboard (hereinafter referred to as ONT-DIF) 20D that are connectedbetween the switch 21 and the optical interface 22. The ONT-UIF 20U isan interface board for handling upward data transmitted from userterminals and to be forwarded toward a wide area network via the OLT 1.The ONT-DIF 20D is an interface board for handling downward dataforwarded from a wide area network toward user terminals via the OLT 1.

The OLT 1 comprises a plurality of PON interface boards (hereinafterreferred to as PON-IFB) 10 (10-1 to 10-m) each accommodating the trunkoptical fiber 700 (700-1 to 700-m), and a switch (or router) 12connected to the PON-IFBs.

The PON-IFB 10 includes an optical interface 11 connected to the trunkoptical fiber 700, an OLT upward interface board (hereinafter referredto as OLT-UIF) 10U and an OLT downward interface board (hereinafterreferred to as OLT-DIF) 10D, each of which is connected between theoptical interface 11 and the switch 12.

To the switch 12 (or router), an access network (local IP network)toward an ISP (Internet Service Provider) network and a wide areanetwork is connected. However, the switch may accommodate a relaynetwork connected to a specific site such as enterprise. It is assumedhere that the switch 12 is composed of a layer 2 switch. In this case,the function of the switch 12 differs depending on the standard of thePON system and the type of data to be transmitted. For example, in thecase where an upward received frame is an Ethernet frame as in the PONsystem of Non-patent Document 4, the switch 12 performs headerinformation processing on the received frame, and control of forwardingthe received frame to an outgoing route. In the case where an upwardreceived frame is a TDM (Time Division Multiplexing) frame as G-PONdescribed in Non-patent Documents 1 to 3, the switch 12 performsencapsulation processing to convert the received frame into an Ethernetframe, and processing for forwarding the received frame to an outgoingroute.

At the place of switch 11, it is able to introduce the processing oflayer 3 protocol of the OSI, and further processing of higher-levellayers including IGMP (Internet Group Management Protocol) proxy and MLD(Multicast Listener Discovery) proxy that are associated with firmware.As the present invention relates to signal multiplexing on the opticalfiber connecting the ONTs and the OLT, however, detailed descriptions onthe functions of the SW 12 are omitted. In the following embodiments, itis assumed that Ethernet (registered trademark) is applied, for example,as protocols from physical layer to transport layer of subscriber lineside UNI (User Network Interface) of the ONT 2.

In FIG. 1, CDMA upward signals are transmitted from the ONT 2-1 to thebranched optical fiber 710-1 by carriers of wavelengths λ(n1-s) toλ(n1-e), CDMA upward signals are transmitted from the ONT 20-2 to thebranched optical fiber 710-2 by carriers of wavelengths λ(n2-s) toλ(n2-e), and CDMA upward signals are transmitted from the ONT 2-k to thebranched optical fiber 710-k by carriers of wavelengths λ(nk-s) toλ(nk-e). These carriers are wavelength-multiplexed on the trunk fiber700 and arrive at the OLT 1. On the other hand, CDMA downward signalsare transmitted by carriers of λ(n1-s) to λ(nk-e) from the OLT 1. Thesedownward signals are broadcast from the trunk fiber 700 to each branchedoptical fiber 710.

Here, the wavelengths λ(n1-s) to λ(n1-e), λ(n2-s) to λ(n2-e), andλ(nk-s) to λ(nk-e) are merely schematically shown to indicate that adifferent carrier wavelength is applied for a different branched fiber.Each ONT 2 uses carrier wavelengths and spreading codes specified fromthe OLT 1 to transmit upward data. The number of wavelengths (or carrierfrequencies) to be used by each ONT 2 may be plural in some cases, butmay be one in other case.

FIG. 2 shows a construction example of the upward interface board(ONT-UIF) 20U in the ONT 2 (2-1 to 2-k).

The ONT-UIF 20U includes a code spreading unit 210, a multiplexing unit220, an optical transmitting unit 230, and a control unit 250. Thecontrol unit 250 is provided with a flow ID table 251 indicating, asshown in FIG. 4(A), the correspondence between a flow identifier (flowID) 2511 and a spreading code number (code-division spreader ID) 2512,and a wavelength allocation table 252 indicating, as shown in FIG. 4(B),the correspondence between a spreading code number (code-divisionspreader ID) 2521 and a carrier length (optical transmitter ID) 2522. Inorder to save memory capacity, however, the flow ID table 251 and thewavelength allocation table 252 may be unified so that thecorrespondence of the flow identifier 2511 to the spreading code number2512 (2521) and carrier wavelength 2522 is indicated by a single table.

The optical transmitting unit 230 includes a plurality of opticaltransmitters 231 (231-1 to 231-y) different in carrier wavelength (laserwavelength) identified by the optical transmitter ID 2522. In thefollowing embodiments, when the number 2521 of spreading code to beapplied to a transmission frame is identified by the flow ID table 251,the carrier wavelength of the transmission frame is determined by thewavelength allocation table 252, and an optical transmitter forconverting a CDMA spread signal into an optical signal is identified.

The code spreading unit 210 includes a reception processing unit 211connected to the SW 21, and a plurality of code-division spreaders 212(212-1 to 212-x) connected to the reception processing unit 211. Uponreceiving an Ethernet frame transmitted from each terminal through theSW 21, the reception processing unit 211 extracts a VLAN-ID (hereinafterreferred to as VID) from the header of received frame, searches the flowID table 251 for a spreading code number j corresponding to the VID, anddistributes the received frame to a code-division spreader 212-jcorresponding to the spreading code number j. The VID may be a valuesuch as the user ID of the source of the received frame, the service IDto which the frame belongs, etc.

The code-division spreader 212-j converts the received frame into ahigh-speed spread spectrum signal having a chip rate of spreading codeby spreading the received frame (Ethernet frame) having a symbol ratewith the spreading code, and outputs it to the multiplexing unit 220.The multiplexing unit 220 selectively supplies each of spread signalsoutputted from the code-division spreaders 212-1 to 212-x to any one ofthe optical transmitters 231-1 to 231-y according to the wavelengthallocation table 252. However, when the number of effective stateoptical transmitters in the optical transmitting unit 230 is one, allspread signals outputted from the code-division spreaders 212-1 to 212-xare transmitted by one carrier.

In the case where the optical transmitting unit 230 is provided with aplurality of optical transmitters 231-1 to 231-y, lasers havingdifferent oscillation frequencies are used for the optical transmitters.When a plurality of spread signals are multiplexed on a specificcarrier, the plurality of spread signals may be linearly added andconverted into a multi-level rectangular wave before being supplied tothe optical transmitters. In this case, the code-division spreaders areclock-synchronized with each other so that the linearly added signalscan be correctly transmitted.

In FIG. 1, carriers of different range are allocated to each ONT so thatthe ONT 2-1 performs wavelength division multiplexing of CDMA spreadsignals in the range of carrier wavelengths λ(n1-s) to λ(n1-e), the ONT20-2 in the range of carrier wavelengths λ(n2-s) to λ(n2-e), and the ONT2-k in the range of carrier wavelengths λ(nk-s) to λ(nk-e). As forspreading code, codes of different range are allocated to each ONT.

FIG. 3 shows a construction example of OLT-UIF 10U.

The OLT-UIF 10U comprises an optical receiver (O/E converter) 110,despreading unit 120, and transmission processing unit 130. CDMA spreadsignals wavelength-division-multiplexed on the trunk optical fiber 700are inputted to the optical receiver 100 through the optical interface11. The optical receiver 100 converts a received optical signal into anelectric signal, and outputs a multi-level rectangular wave signal tothe despreading unit 120. The multi-level rectangular wave signal isgenerated by linearly adding all spread signals contained in thereceived optical signal. Here, the optical receiver 100 is composed ofan optical receiving device capable of converting CDMA optical signalsinto the electric signal within the range of carrier frequencies λ(n1-s)to λ(nk-e).

The despreading unit 120 comprises a plurality of despreaders 121 (121-1to 121-x) different in the spreading code to be applied. Each of thedespreaders 121 performs despread processing on the multi-levelrectangular wave outputted from the optical receiver 100, by using aspreading code (code 1, code 2, . . . code x) peculiar thereto. Here,the despreading refers to detecting the correlation between the receivedsignals and the spreading code. For example, the despreader 121-1comprises a matched filter between the received signals and code “Code1” that exhibits high correlation as to a signal component having beenspread with “Code 1” at a transmission side, but further weakens thesignal strength by dispreading as to signal components having beenspread with the other codes. Therefore, by integrating the output of thematched filter throughout a code period of chip rate, data of symbolrate having been spread with “Code 1” at the transmission side can bereproduced.

The transmission processing unit 130 performs header processing on eachreceived frame outputted at the symbol rate from the despreading unit120 as required, and forwards it to the switch 12. In this case, theheader processing includes, for example, attaching a VLAG tag or MPLSlabel to the header, header conversion, partial deletion of headerinformation, and the like.

FIG. 5 shows spread signals inputted to the multiplexing unit 220 of theONT-UIF 20U (FIG. A and B), and a multi-level rectangular wave generatedby performing linear addition processing on them (FIG. C).

FIGS. (A) and (B) show chip rate signals S1 and S2 having been spreadwith different spread signals, respectively. When the signals S1 and S2are linearly added in the state in which clock timings of twocode-division spreaders for outputting the signals S1 and S2 aresynchronized with each other, a multi-level rectangular wave signal S3is obtained as shown in FIG. (C). In the case of transmitting thesignals S1 and S2 with the same carrier, they are supplied to theoptical transmitter after converting into the signal S3, and convertedinto an optical signal of predetermined carrier wavelength by using thelinearity of laser output signal strength.

The optical receiver 110 of OLT-UIF 10U converts CDMA optical signalswavelength-division-multiplexed with a plurality of carriers into anelectric signal. At this time, by converting optical signals on aplurality of carriers into an electric signal at the same time in clocksynchronization with the optical signals transmitted at the chip rate,the output signal waveform of the optical receiving device becomes amulti-level rectangular wave like FIG. (C). By multiplying themulti-level signal by spreading codes Code 1 to Code x by thedespreaders 121-1 to 121-x, symbol rate data corresponding to thespreading codes can be restored.

In the present embodiment, usable carrier wavelengths and spreadingcodes are allocated in a different range to each ONT, according to theflow ID table 251 and the wavelength allocation table 252. However, thecorrespondence between carrier wavelengths and spreading codes are freeamong the ONTs. All spreading codes (spread signals) maybe associatedwith different carriers, and a plurality of spreading codes may beassociated with an identical carrier.

By applying different spreading code to each ONT, a plurality ofcarriers can be formed logically in an optical fiber section so thatflow identification is made possible at a reception (OLT) side.Moreover, by transmitting spread signals with carrier wavelengthsdifferent for each ONT, physical signal interference can be suppressedin an optical fiber section.

FIG. 6 shows the allocation of carrier wavelengths (carrier frequencies)to ONTs and flow IDs.

In the present embodiment, a plurality of carriers (laser beams) 501-1to 501-N having their peaks at mutually different frequencies 511-1 to511-N, respectively, are defined in an optical bandwidth 500 receivableby the optical receiver 110 provided in the OLT-UIF 10U. Control oflaser frequencies (wavelengths) will be detailed later. The frequencies511-1 to 511-N are arranged at a fixed frequency interval Δf.

The frequency interval Δf of mutually adjacent laser beams is determinedto be a sufficient value so that the laser beams cause no interferenceon the branched optical fibers and the trunk optical fiber andinterference components (beat noises) are not detected by the receivers.Specifically, the frequency interval Δf may be equal to or greater thanthe bandwidth of CDMA spread signals that can be received by the opticalreceiver 110. For example, in the transmission of CDMA optical signalwhose chip rate is 10 Gbps, a frequency interval of 10 GHz or more maybe secured between lasers having adjacent wavelengths.

In the lower portion of FIG. 6, the relationships between carriers(laser frequencies) and spreading codes allocated to ONTs are shown.Here, frequencies 511-1 and 511-2 (laser beams 501-1 and 501-2) servingas carriers are allocated to ONT #1, a frequency 511-3 (laser beam501-3) is allocated to ONT #2, and a plurality of frequencies (laserbeams 501-4, . . . ) beginning with a frequency 511-4 are allocated toONT #3. Further, spreading codes #1 and #3 to be used by ONT #1 areallocated to the frequency 511-1, and spreading code #2 to the frequency511-2. Spreading codes #5 to #7 to be used by ONT #2 are allocated tothe single frequency 511-3. In each ONT, the correspondence betweenspreading codes and frequencies is stored in the wavelength allocationtable 252 shown in FIG. 4.

FIG. 7 shows a construction example of the OLT downward interface board(OLT-DIF) 10D.

The OLT-DIF 10D comprises a control unit 150, code spreading unit 160,multiplexing unit 170, and optical transmitting unit 180.

The control unit 150 is provided with a flow ID table 151 and awavelength allocation table 152. The flow ID table 151 indicates, forexample, as shown in FIG. 9, the correspondence among a flow identifier(flow ID) 1511, a spreading code number (code-division spreader ID)1512, and an ONT identifier (ONT-ID) 1513. The wavelength allocationtable 152 indicates, like the ONT wavelength allocation table 252 shownin (B) of FIG. 4, the correspondence between the spreading code number(code-division spreader ID) and the carrier wavelength (opticaltransmitter ID). As described in the ONT-UIF 20U, the flow ID table 151and the wavelength allocation table 152 may be unified into one table.

The optical transmitting unit 180 includes a plurality of opticaltransmitters (laser elements) 181 (181-1 to 181-y) different inoscillation frequency (laser wavelength λ). The wavelength of eachoptical transmitter is determined so as to secure a sufficientwavelength interval between adjacent wavelengths so that a plurality oflaser beams do not interfere with each other on the optical fiber.

The code spreading unit 160 comprises a reception processing unit 161connected to the switch 12, and a plurality of code-division spreaders162 (162-1 to 162-j) connected to the reception processing unit 161.Upon receiving a frame from the switch 11, the reception processing unit161 adds a header including an ONT identifier to the received frame byreferring to the flow ID table 151, and after that, distributes thereceived frame to a proper code-division spreader.

Specifically, the reception processing unit 161 extracts a data flowidentifier (flow ID) such as the user ID of a transmitting source orservice ID and the like, from the header of the received frame, andsearches the flow ID table 151 for a spreading code number i and ONTidentifier that correspond to the flow ID. After converting the receivedframe into a PON frame having the ONT identifier, the receptionprocessing unit 161 forwards it to a code-division spreader 162-iidentified by the spreading code number i. However, the conversion ofthe received frame into a PON frame may be performed, for example, byreferring to a specific route table indicating the correspondencebetween header information of the received frame and the identifier ofONT to be the forwarding destination of the received frame.

The code-division spreader 162-i converts the received frame into awide-band spectrum spread signal by spreading each symbol of thereceived frame with a spreading code having a rate (chip rate) fasterthan the symbol rate of the received frame. The spread signal outputtedfrom the code-division spreader 162-i is forwarded to the multiplexingunit 170. The multiplexing unit 170 distributes the spread signalsupplied from the code-division spreader 162-i to an optical transmitter181-j corresponding to the spreading code number i.

Specifically, the multiplexing unit 170 specifies a carrier frequency(wavelength λ) corresponding to the spreading code number i by referringto the wavelength allocation table 152, and supplies the spread signalto an optical transmitter (laser element) 181-k which oscillates at thespecified frequency. In the case where the wavelength allocation table152 correlates a plurality of spreading codes with one carrierfrequency, the multiplexing unit 170 linearly adds outputs from aplurality of code-division spreader to be superimposed on the samecarrier, and supplies a spread signal converted into a multi-levelrectangular wave to the optical transmitter having the specified carrierfrequency.

In the case where a control message is transmitted from the control unit150 of the OLT 1 to the OLT 2 (2-1 to 2-k), the control message iscode-spread by a specific code-division spreader, for example, acode-division spreader 162-j, and supplied to a specific opticaltransmitter through the multiplexing unit 170.

By the above-described construction, CDMA signalswavelength-division-multiplexed with carriers of wavelengths λ(n1-s) toλ(nk-e) are transmitted from the PON-IFB 10-1 of OLT 1 to the trunkoptical fiber 700-1. These CDMA signals are diverged to the branchedfibers 710-1 to 710-k by the optical splitter 800-1, and arrive at allONTs 2-1 to 2-k.

FIG. 8 shows the construction of the ONT downward interface board(ONT-DIF) 20D.

The ONT-DIF 20D includes an optical receiver 260, a despreader unit 270,and a transmission processing unit 280. The despreader unit 270 includesa plurality of despreaders 271-1 to 271-j each of which despreads areceived signal with a spreading code different to each other. In eachONT, however, among the plurality of despreaders 271-1 to 271-j, only aspecific number of despreaders having specific spreading codes allocatedin advance are made effective (active despreaders), and the otherdespreaders are standby despreaders.

The optical receiver 260 receives CDMA spread signals, which weretransmitted from the OLT 1 by being wavelength division multiplexed,through the branched fiber 710 and the optical interface 22, andconverts the spread signals into an electric signal of multi-levelrectangular wave by linearly adding all the spread signals. In thepresent embodiment, the optical receiver 260 is constructed by oneoptical receiving device (photodiode: DA).

The electric signal of multi-level rectangular wave outputted from theoptical receiver 260 is inputted in parallel to the plurality ofdespreaders 271-1 to 271-j in the despreading unit 270. To thesedespreaders, spreading codes Code 1 to Code j that were applied to thetransmission data by the code spreading unit 160 of the OLT-DIF 10D areallocated. Each despreader multiplies the electric signal of multi-levelrectangular wave supplied from the optical receiver 260 by a specificspreading code, and outputs, as the symbol value of data flowcorresponding to the spreading code, a binary decision result of anintegral value during a time period corresponding to a symbol rate.

By using one of the despreaders 271-1 to 271-j, for example, adespreader 271-j having the same spreading code Code j as thecode-division spreader 162-j, with which the OLT 1 performs codespreading of control frames, for use in control frame reception andforwarding a received frame reproduced by the despreader 271-j to thecontrol unit 250, control information can be distributed from the OLT 1to the ONU 2.

The transmission processing unit 280 determines header information ofPON frames each composed of a symbol sequence reproduced by each of thedespreaders 271-1 to 271-j, and discards PON frames addressed to otherstations. Upon receiving a PON frame addressed to its own station, thetransmission processing unit 280 performs header processing, thereby toforward a control frame to the control unit 250 and a user frame to theswitch unit 21. The frame header processing includes, for example,elimination of a PON header, and adding/conversion/deletion of VLAN tagor MPLS label, and the like. The switch unit 21 forwards each framereceived from the transmission processing unit 280 to any one ofsubscriber lines, which includes the switch connection line and LANconnection line, specified in accordance with the header information.

In order to allocate usable carrier wavelengths (laser wavelengths) andusable spreading codes from the OLT 1 to the ONT 2, the control unit 150of OLT-DIF 10D stores carrier frequencies (laser wavelengths) andspreading codes having been allocated to each ONT. Each PON interfaceboard 10 can allocate a plurality of carrier frequencies (laserwavelengths) to the same ONT and a plurality of spreading codes to thesame ONT. In this case, same carrier frequency should be avoided frombeing duplicately allocated to a plurality of ONTs. This is similar tospreading codes; a spreading code having been already allocated to anyONT should be avoided from being allocated to other ONTs.

Carrier frequencies (laser wavelengths) and spreading codes having beenallocated to each ONT are stored, for example, in a management table 153in association with ONT identifiers. The management table 153 is usefulto grasp the operational status of the system, not only in the casewhere carrier frequencies (laser wavelengths) and spreading codes arestatically allocated to each ONT, but also in the case where carrierfrequencies (laser wavelengths) and spreading codes are dynamicallyallocated when ONT was started.

FIG. 10 shows another embodiment of ONT-UIF 20U.

In the present embodiment, carrier wavelengths of the opticaltransmitters 231 (231-1 to 231-y) are controlled by using thetemperature dependency of laser beams. However, the oscillationfrequencies of laser elements may be controlled by parameters other thanthe temperature.

The optical transmitters 231 of the present embodiment comprises a laser232, a modulation circuit 233, a temperature controller 234 connected tothe laser 232, and a control information register 235. The temperaturecontroller 234 obtains a target temperature form the register 235, andautomatically controls so that the laser operates at the targettemperature, whereby the laser 232 generates an optical CDMA signal(modulation light of spread signal) at a predetermined wavelength(carrier frequency) determined by the target temperature. Output lightfrom the laser 232 is transmitted to the branched fibers 710 through theoptical interface 22.

Loading of control information to each register 234 is performed by thecontrol unit 250. This configuration becomes effective when a laserwavelength (carrier frequency) usable in the optical transmitter 230 isdesignated from the OLT 1 to each OLT 2. The wavelength usable in theoptical transmitter 230 is notified by a downward control message. Ifthe wavelength is specified, a target temperature of the laser isuniquely determined.

The control unit 250 of each ONT stores the wavelengths designated fromthe OLT 1 in the wavelength management table 253 in association with theidentifiers (ID) of optical transmitters, and sets a target temperaturedetermined by the wavelength to the register 235 of the opticaltransmitter.

FIG. 11 shows an example of the wavelength management table 253.

The wavelength management table 253 comprises a plurality of entrieseach storing an optical transmitter ID 2531, and a wavelength 2532specified from the OLT 1. The value of frequency may be stored insteadof the wavelength 2532. The control unit 250 can uniquely determine atarget temperature of the laser if a wavelength is specified. Therefore,target temperature 2533 corresponding to the wavelength 2532 may bestored in each entry so that the control unit 250 sets the targettemperature 2533 one after another to the control information register235 of the optical transmitter indicated by the optical transmitter ID2531.

FIG. 12 shows further another embodiment of ONT-UIF 20U.

In the present embodiment, an external resonance laser 236 using fibergrating (FG) 237 is applied to the optical transmitter 231. The externalresonance laser 236 can cause the optical transmitter 231 to oscillateat a desired wavelength by adjusting the oscillation wavelengthaccording to the position of the fiber grating 237. However, in theexternal resonance laser 236 using the fiber grating (FG) 237, if itsoscillation frequency has been adjusted once to a desired value when theoptical transmitter was assembled, it becomes difficult to change thewavelength dynamically by a control signal supplied from the outside atthe time of starting the ONT like the temperature control type lasers.

Therefore, in the present embodiment, a plurality of opticaltransmitters 231-1 to 231-y different in their oscillation wavelengthare provided in advance in the optical transmitter 230 so that opticaltransmitters each having wavelength allocated from the OLT 1 are madeeffective based on the wavelength allocation table 252 and multiplexingof code spread signals is performed.

FIG. 13 shows further another embodiment of ONT-UIF 20U to which theexternal resonance laser 236 is applied.

In the present embodiment, instead of dynamic carrier wavelengthselection with reference to the wavelength allocation table 252, themultiplexing unit 220 connects fixedly between the code-divisionspreaders 212 and the optical transmitters 231 by adders 221 (221-1 to221-w). Therefore, at the time when the reception processing unit 211has allocated a spreading code to a received frame, a carrier frequencyto which the spreading code should be superimposed is determined.

In the above-described embodiment, the reception processing unit 211(161) of the code spreading unit extracts a VLAN identifier (VID) as aflow ID from the received frame, searches the flow ID table 251 (151)for a spreading code number corresponding to the flow ID, and determinesa code-division spreader 212 (162) to process the received frame.However, spreading code does not always need to be defined individuallyfor each of VIDs on the transmission side. For example, in the casewhere a plurality of VLANs are used by the same user, spreading codesmay be allocated for each user. Conversely, in the case where a VLAN isallocated to each group comprising a plurality of users, for example,when a closed area network is constructed for each enterprise site, aspreading code applied to the received frame is conceivably used as asub-parameter for flow identification to identify a user. Therefore, thecorrespondence between flow IDs and spreading code numbers variesdepending on the configuration of networks and communication services.

Although optical CDMA is applied to a PON system in the above-describedembodiments, optical CDMA of the present invention is also applicable tocommunication systems other than PON.

For example, in an optical multiplexing transmission apparatus used in acore network or an optical switch used in an access network and a metronetwork, it is able to transmit a plurality of spread signals in awavelength-multiplexed form and to receive them by using a singleoptical receiving device at the receiving side, like the above-describedONT and OLT.

FIG. 14 shows the construction of a general optical signal multiplexingapparatus to which optical CDMA of the present invention is applied.

The basic construction of a transmission interface 200A and a receptioninterface 200B provided in each of communication apparatuses 3A and 3Bconnected through an optical fiber 750 is the same as that of ONT-UIF20U and OLT-UIF 10U of the PON shown in FIG. 2. In the case of generaloptical signal multiplexing apparatus, there is no branch on the opticalfiber between the communication apparatuses, different from the case ofPON, it is sufficient for a spreading code on a carrier wavelength tosecure orthogonality for each optical fiber (route).

The present invention is also applicable to an optical communicationapparatus, such as a wholly-optical switch that is currently developedwith miniaturizing of laser oscillation devices, in addition to PON asan existing optical communication system and an optical multiplexingtransmission apparatus in core networks. Moreover, by combining withTDM, it is able to further increase the number of users to bemultiplexed on the same optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1]

A figure showing the construction of a PON system to which the presentinvention is applied, and carrier frequencies in the optical fibersection.

[FIG. 2]

A figure showing a construction example of an upward interface board(ONT-UIF) 20U provided in the ONT 2 of FIG. 1.

[FIG. 3]

A figure showing a construction example of an upward interface board(ONT-UIF) 10U provided in the ONT 1 of FIG. 1.

[FIG. 4]

Figures showing a flow ID table 251 (FIG. A) and wavelength allocationtable 252 (FIG. B) provided in a control unit 250 of ONT-UIF 20U.

[FIG. 5]

Figures showing spread signals to be inputted to a multiplexing unit 220of ONT-UIF 20U (FIGS. A and B) and a multi-level rectangular wavegenerated by linear addition processing of the spread signals (FIG. C).

[FIG. 6]

A figure showing allocation of carrier frequencies to ONTs and flow IDs.

[FIG. 7]

A figure showing a construction example of a downward interface board(OLT-DIF) 10D provided in the ONT 1.

[FIG. 8]

A figure showing a construction example of a downward interface board(ONT-DIF) 20D provided in the ONT 2.

[FIG. 9]

A figure showing the contents of a flow ID table 151 provided in acontrol unit 150 of OLT-DIF 10D

[FIG. 10]

A figure showing another embodiment of ONT-UIF 20U.

[FIG. 11]

A figure showing the contents of a wavelength management table 253provided in a control unit 250 of FIG. 10

[FIG. 12]

A figure showing further another embodiment of ONT-UIF 20U.

[FIG. 13]

A figure showing further another embodiment of ONT-UIF 20U.

[FIG. 14]

A figure showing the construction of a general optical signalmultiplexing apparatus to which optical CDMA of the present invention isapplied.

DESCRIPTION OF THE REFERENCE NUMERALS

1: OLT, 2: ONT, 10: PON interface board, 11: Optical interface, 10U:OLT-UIF, 10D: OLT-DIF, 12: Switch, 20U: ONT-UIF, 20D: ONT-DIF, 21:Switch, 22: Optical interface, 110: Optical receiver, 120: Despreadingunit, 121: Despreader, 130: Transmission processing unit, 160: Codespreading unit, 161: Reception processing unit, 162: Code-divisionspreader, 170: Multiplexing unit, 180: Optical transmitting unit, 181:Optical transmitter, 150: Control unit, 151: Flow ID table, 152:Wavelength allocation table, 153: Management table, 210: Code spreadingunit, 211: Reception processing unit, 212: Code-division spreader, 220:Multiplexing unit, 230: Optical transmitting unit, 231; Opticaltransmitter, 250: Control unit, 251: Flow ID table, 252: Wavelengthallocation table, 260: Optical receiver, 270: Despreading unit, 271:despreader, 280: Transmission processing unit, 300: Switch, 500:Terminal

1. (canceled)
 2. An optical communication system comprising an officeside apparatus (OLT) connected to a trunk optical fiber, and a pluralityof subscriber connecting apparatuses (ONTs) connected to branchedoptical fibers diverged from said trunk optical fiber, wherein each ofsaid ONTs comprises: a plurality of code-division spreaders different intheir spreading code; a reception processing unit that selectivelydistributes transmission data to the plurality of code-divisionspreaders; at least one optical transmitter that transmits a signalspread with a code to said branched optical fiber as an optical signalof a carrier wavelength different from that of the other opticaltransmitters; and a signal multiplexing unit that selectively suppliesoutput of the plurality of code-division spreaders to the opticaltransmitter, the ONT converts the transmission data into a CDMA signalby the code-division spreader having a spreading code allocated from theOLT, and converts the CDMA signal into an optical signal by the opticaltransmitter having a carrier wavelength allocated from the OLT, andwherein said OLT comprises: an optical receiver that receives awavelength-division-multiplexed CDMA signal from said trunk opticalfiber; and a plurality of despreaders connected to the optical receiverand different in their spreading code, each of the despreadersreproducing a CDMA signal corresponding to its spreading code from anoutput signal of said optical receiver.
 3. (canceled)
 4. The opticalcommunication system according to claim 2, wherein, each of said ONTsincludes a plurality of optical transmitters different in their carrierwavelength, and said signal multiplexing unit selectively suppliesoutputs of the plurality of code-division spreaders to the plurality ofoptical transmitters; and wherein each of said ONTs converts saidtransmission data into a CDMA signal by said code-division spreaderhaving a spreading code allocated from the OLT, and converts the CDMAsignal into an optical signal by said optical transmitter having acarrier wavelength allocated from said OLT.
 5. The optical communicationsystem according to claim 2, wherein said OLT is provided with amanagement table for storing a carrier wavelength and a spreading codeallocated to each of said ONTs, in association with an identifier of theONT.
 6. The optical communication system according to claim 2, whereineach of said optical transmitters is composed of a laser elementadjustable its oscillation wavelength, and wherein each of said ONTscontrols each of said laser elements so that the optical transmitter hasa carrier wavelength allocated in advance.
 7. The optical communicationsystem according to claim 5, wherein each of said ONTs changes saidoscillation wavelength by controlling temperature of each of said laserelements.
 8. The optical communication system according to claim 4,wherein each of said ONTs has a wavelength allocation table indicatingthe correspondence between spreading codes and carrier wavelengths, andsaid signal multiplexing unit correlates said code-division spreadersand said optical transmitters according to the wavelength allocationtable.
 9. The optical communication system according to claim 2, whereineach of said ONTs has a flow identifier table indicating thecorrespondence between flow identifiers of transmission data andspreading codes, and said reception processing unit correlates saidtransmission data and said code-division spreader according to the flowidentifier table.
 10. The optical communication system according toclaim 4, wherein said OLT is provided with a management table forstoring a carrier wavelength and a spreading code allocated to each ofsaid ONTs, in association with an identifier of the ONT.
 11. The opticalcommunication system according to claim 4, wherein each of said opticaltransmitters is composed of a laser element adjustable its oscillationwavelength, and wherein each of said ONTs controls each of said laserelements so that the optical transmitter has a carrier wavelengthallocated in advance.
 12. The optical communication system according toclaim 10, wherein each of said ONTs changes said oscillation wavelengthby controlling temperature of each of said laser elements.
 13. Theoptical communication system according to claim 4, wherein each of saidONTs has a flow identifier table indicating the correspondence betweenflow identifiers of transmission data and spreading codes, and saidreception processing unit correlates said transmission data and saidcode-division spreader according to the flow identifier table.
 14. Theoptical communication system according to claim 5, wherein each of saidONTs has a flow identifier table indicating the correspondence betweenflow identifiers of transmission data and spreading codes, and saidreception processing unit correlates said transmission data and saidcode-division spreader according to the flow identifier table.
 15. Theoptical communication system according to claim 6, wherein each of saidONTs has a flow identifier table indicating the correspondence betweenflow identifiers of transmission data and spreading codes, and saidreception processing unit correlates said transmission data and saidcode-division spreader according to the flow identifier table.
 16. Theoptical communication system according to claim 7, wherein each of saidONTs has a flow identifier table indicating the correspondence betweenflow identifiers of transmission data and spreading codes, and saidreception processing unit correlates said transmission data and saidcode-division spreader according to the flow identifier table.
 17. Theoptical communication system according to claim 8, wherein each of saidONTs has a flow identifier table indicating the correspondence betweenflow identifiers of transmission data and spreading codes, and saidreception processing unit correlates said transmission data and saidcode-division spreader according to the flow identifier table.